1. Field of the Invention
The present invention relates to a digital radio communication system and in particular, to a method for receiving digital radio signals and a digital radio signals receiver which can minimize power consumption and maintain good voice quality. This invention is useful for digital mobile radiotelephones, digital cellular radiotelephones, digital cordless telephones, Personal HandyPhone system and the like.
2. Description of the Prior Art
FIG. 3 is a block diagram which shows the main parts of a conventional digital radio signal receiver.
Digital radio signal receiver 300 comprises radio frequency (RF) part 11 which amplifies radio frequency signals received by an antenna and converts them to intermediate frequency signals using predetermined frequency signals from a local oscillator, intermediate frequency (IF) part 12 which amplifies the intermediate frequency signals, differential detector part 13 was designed for demodulation of differential quadrilateral phase shift keying (DQPSK), parallel-to-serial (P/S) converter part 14 which converts parallel signals to a serial signal, framing part and voice decoder part 15 which outputs error information and extracts voice data from the serial signal, speaker 16 which produces a voice based on the voice data, microprocessor unit (MPU) 17 which performs a retransmission request based on the error information, bit timing recovery (BTR) part 18 which supplies regenerated synchronous clock signal to P/S part 14, and clock generator 19 which produces a sampling clock 32f to differential detector part 13 and synchronous clock to the BTR part 18.
A system similar to digital radio signal receiver 300 is disclosed for example in "Performance of .pi./4-Shift QPSK Baseband Differential Detector; NTT Radio Communication Systems Laboratories; The National Conference of the Institute of Electronics and Communications Engineers, Autumn 1990".
In conventional digital radio signal receiver 300, the frequency of the sampling clock 32f fed to differential detector part 13 is 32 times higher than a symbol rate frequency, for example, 192 kHz. In this case, as the sampling rate frequency, for example, 6.144 MHz is high enough, a good voice quality is obtained when used for digital mobile radiotelephones, digital cellular radiotelephones, digital cordless telephones and the like.
However, when the sampling rate is maintained at the high frequency, a power consumption in differential detector part 13 increases and arises a problem arises that batteries contained within digital mobile radiotelephones, digital cellular radiotelephones, digital cordless telephones and the like are consumed faster. On the other hand, if the sampling rate frequency is maintained low, power consumption in the differential detector part 13 decreases. However, the voice quality becomes worse, because a stability against a fading, multipath and similar effects becomes low.